Method and device for monitoring the regeneration of a pollution-removal system

ABSTRACT

A method for monitoring regeneration of a pollution-removal system, relying on introduction of fuel into exhaust gases by delayed injections of fuel into certain combustion chambers of an engine and/or direct injections into an exhaust system upstream of a filter depending on an inlet temperature of the system. Introduced fuel delivery is assigned to direct injections into the exhaust system and/or to delayed injections into the combustion chamber according to the value of the wall temperature of the exhaust system.

The present invention is in the field of internal combustion engines and more particularly to diesel-type engines, since they emit particulates. In fact, the present invention relates in particular to the management of particulate filters or PF.

It is applicable in particular to any vehicle equipped with a particulate filter, but also in the case of use of an additional injector for strategies of purging or desulfating a nitrogen oxide trap (NOx trap).

In contrast to a traditional oxidation catalyst, these systems function in discontinuous or alternating manner, meaning that, in normal operation, they trap the pollutants in order to treat them exclusively during regeneration phases. To be regenerated, these filters or traps require specific modes of combustion, in order to guarantee the necessary temperature and/or richness levels.

To regenerate the particulate filters, it is possible to initiate one or more delayed injections into the combustion chambers of the engine, after the top dead center (TDC), during the expansion phase, these injections having the effect of raising the temperature of the exhaust gases. The gas-oil injected long after TDC burns not in the combustion chamber but in the catalytic part of the exhaust line. With the same objective of lowering the pollutant emissions, it is in fact possible to dispose, in addition to the PF, either a diesel oxidation catalyst (DOC) in the exhaust line, upstream from the PF, or a catalytic material (such as platinum) directly within the PF. It is on these catalytic sites that the HC and CO of the delayed injections are oxidized, raising the temperature of the gases.

Finally, by increasing the flow by much later post-injection, high concentrations of HC and CO are emitted at the engine outlet. In the oxidation catalyst, these reducing agents react with the oxygen present in the exhaust gases, generating heat, which contributes to raising the temperature of the exhaust gases at the inlet of the particulate filter.

In this way the regeneration of a particulate filter is able to use the heat produced by an oxidation catalyst generally positioned upstream from the particulate filter, and that of the catalytic phase with which the particulate filter is coated. This achieves the function of oxidation of the hydrocarbons and carbon monoxide not treated by the oxidation catalyst. It is also able to use the heat produced by the phase of oxidation of the catalytic particulate filter when there is no oxidation catalyst upstream therefrom.

Activation of the different means for assisting regeneration is generally slaved to the engine control calculator, which determines, as a function of several parameters, including the soot load of the particulate filter, the instant and duration of regeneration as well as the injection parameters during this phase.

To improve regeneration efficiency, however, it is necessary to achieve a filter internal temperature that favors oxidation of soot (570-650° C.) and thus lies above the normal exhaust temperature, and to do so regardless of the engine operating point. Similarly, to optimize the treatment of all of the pollutants, it is necessary to manage the phases of storage and regeneration of these traps as effectively as possible. These operations therefore make it necessary to control the temperature at the inlet to the particulate filter, at the instant of the regeneration phases, and the dilution due to post-injection.

At present, the heat necessary for regeneration of the particulate storage elements is generated by means of supplementary injections, either during the cylinder expansion phase or directly into the exhaust line. Regulation of injection is generally achieved by feedback of the outlet temperature T_(DOCO) of the diesel oxidation catalyst by means of a PID (proportional-integral-derivative) controller, which applies a calculated correction to regulate this temperature.

The two actuators available to achieve the temperature rise expected in the catalytic phase of the exhaust line are not equal with regard to the criterion of dilution of fuel in the lubricating oil.

The use of post-injection into the cylinder creates a large extra cost in terms of diluent material, whereas the use of direct injection into the exhaust may allow more flexibility in perfecting the system in this regard.

The objective of the present invention is to maximize the particulate filter regeneration performances by giving the injection of reducing agents into the exhaust line priority over post-injection, in order to limit the cost of dilution associated with the use of post-injection.

To this end, it proposes that the introduced fuel flow be assigned either to direct injections into the exhaust line and/or to delayed injections into the combustion chamber, depending on the value of the wall temperature.

Preferably the injection of fuel into the exhaust line is limited to one zone of relatively low loads and to one zone of relatively high loads of the engine, and the flow of fuel injected into the exhaust line is limited to a maximum flow, beyond which the injected fuel would not be completely oxidized therein.

The invention also proposes a device comprising a first temperature transducer upstream from the turbine, an oxidation catalyst, a second temperature transducer that measures the temperature at the inlet of a depollution system, the depollution system and a means for determining the wall temperature of the exhaust line.

Other characteristics and advantages of the invention will become clear upon reading the description hereinafter of a non-limitative embodiment thereof with reference to the drawings, wherein:

FIG. 1 shows an example of application of the invention,

FIG. 2 shows the distribution of injections as a function of the exhaust conditions,

FIG. 3 presents the method for determining the wall temperature,

FIG. 4 is a block diagram of the control system, and

FIG. 5 presents graphs of saturation of the amount of fuel injected into the exhaust line (fifth injector) for three wall temperatures.

FIG. 1 illustrates the application of the invention to a vehicle engine in non-limitative manner. It shows a four-cylinder engine 1, turbine 2 and compressor 3 of a turbocompressor, as well as an EGR loop and its cooler 4. In the exhaust line there is disposed a diesel oxidation catalyst 7 (DOC) followed by a particulate filter 8 (PF). A means 9, known as the fifth injector, for injecting fuel into the exhaust, is disposed upstream from catalyst 7. The different associated transducers are a temperature transducer (T_(ups) _(—) _(t)) 11 upstream from the turbine, a temperature transducer (T_(pfi)) 13 at the particulate filter inlet, a temperature transducer (T_(pfo)) 14 at the particulate filter outlet, an oxygen sensor 16 and a differential pressure transducer 17 or relative pressure transducer between the upstream end of the filter inlet and atmosphere. Lastly, the diagram shows engine intake throttle valve 8, EGR valve 19 and exhaust-line isolating means 21. Associated engine calculator 22 receives and processes the signals emitted by the cited transducers as well as other information items originating from electrical loads 23, from motor fan group 25, from a slave thermostat 26 and from atmospheric temperature and pressure transducers 27, 28.

Within the scope of the invention, however, the supplementary injector or fifth injector 9 positioned in the exhaust line can be placed either upstream or downstream from the turbine, such placement having no effect on the proposed strategy. The device to which the invention relates therefore comprises the following elements: an exhaust injector 9, a first temperature transducer 11 upstream from the turbine, an oxidation catalyst 8, a second temperature transducer 12 measuring the temperature T_(pfi) at the inlet to a depollution system, depollution system 8 and a means for determining the wall temperature T_(wall) of the exhaust line. According to the invention, the wall-temperature means may be a calculating algorithm integrated into the calculator or a wall-temperature transducer (not shown). Finally, depollution system 8 may be either a particulate filter or another system such as a nitrogen oxides trap, and exhaust injector 9 may be disposed upstream or downstream from the turbine.

As indicated hereinabove, the invention provides for distributing the fuel amount Q_(red) for adjusting the desired temperature at the particulate filter inlet between a supplementary injector installed in the exhaust-gas flow and the post-injection.

More precisely, the quantity Q_(red) of reducing agents ordered by the strategy of temperature control at the particulate filter inlet will be assigned prioritarily to the supplementary injector (Q_(5inj)) and/or to the post-injection (Q_(poi)), depending on the instantaneous value of the wall temperature T_(wall) of the exhaust line.

The invention is based on the principle that the exhaust injector cannot be used over the entire range of operation of the engine. In fact, the zone characterized by a weak exhaust-gas flow and a low wall temperature does not permit satisfactory vaporization of the injected fuel. For safety, it may also be preferable to avoid using the exhaust injector in the zones characterized by a strong exhaust-gas flow and an elevated wall temperature, because then the dwell time of the reducing agents in the oxidation catalyst would be too short to permit oxidation of the totality of the reducing agents. According to FIG. 2, the injection of fuel into the exhaust line is therefore used exclusively in certain ranges of operation of the engine, and limited, for example, to one zone of relatively low load and one zone of relatively high load of the engine.

The wall temperature may be determined either by a transducer or by an algorithm integrated into the engine calculator, as a function of different parameters. In order to determine the wall temperature T_(wall), it is in fact possible to use a transducer or a calculating algorithm, for example integrated into the engine control calculator, thus making it possible to obtain an instantaneous value of T_(wall). This temperature is a function of different parameters indicated in FIG. 3, including the exhaust-gas temperature T_(ups) _(—) _(t) upstream from the turbine of a turbocompressor, the water temperature T_(water) of the engine, the exhaust-gas flow Q_(eg) and the air flow Q_(air) (measured at the intake, for example). The algorithm may use all or only some of these parameters, depending on the engine operating point.

The amount Q_(red) of fuel to be injected depends on the wall temperature, on the temperature at the outlet of the diesel oxidation catalyst DOC or on the temperature T_(pfi) at the inlet of the PF and on the engine operating point (exhaust-gas flow). The fuel amount Q_(red) is calculated by means of a module integrated into the engine control calculator. This module, illustrated by FIG. 4, is composed of baseline regulation of the flow of reducing agent to be injected (assumed to be independent of the actuator), mapping of engine speed/torque by operating point, and a correction generated by a correcting means of PID type (proportional-integral-derivative), which correction depends on the difference between the temperature measured at the particulate filter inlet and the setpoint temperature T_(set).

The conversion capacity of the DOC, which depends on the wall temperature and the flow of gases passing over the wall, defines a maximum flow for the fifth injector, beyond which part of the reducing agents injected into the exhaust line will not be oxidized. To allow for this constraint, the invention provides that the fuel flow Q_(5inj) injected into the exhaust line will be limited to a maximum flow Q_(inj max), beyond which the injected fuel would not be completely oxidized therein. More precisely, the fuel is injected prioritarily into the exhaust line as long as the injected flow Q_(inj) is smaller than the maximum flow Q_(inj max) that can be completely oxidized therein.

FIG. 5 illustrates the principle of high saturation of the flow of the fifth injector for different wall temperatures T_(wall1), T_(wall2), T_(wall3). In the two zones in which this injector cannot be used, post-injection will be authorized if the strategy of temperature control at the PF inlet requires production of a temperature rise in the DOC.

When the use of the fifth injector is authorized, it is preferentially saturated, in such a way that the use thereof has priority until saturation occurs, after which any ordered surplus is passed on to post-injection:

if Q _(red) <Q _(5inj) max, then Q _(5inj) =Q _(red) and Q_(poi1)=0

if Q _(red) ≧Q _(5inj) max, then Q _(5inj) =Q _(5inj) max and Q _(poi1) =Q _(red) −Q _(5inj) max.

Thus the fuel surplus Q_(poi) relative to the flow Q_(inj max) that can be oxidized in the exhaust line is introduced by delayed injections into the combustion chambers of the engine. Preferably, engine calculator 22 directs the fuel flow Q_(red) into the dedicated injector of exhaust line 9 until a saturation level of oxidation catalyst 7, before passing on the surplus ordered by regeneration of filter 8 to delayed injections of fuel into the combustion chambers of the engine.

In the case of simultaneous activation of injection into the exhaust and of post-injection, it is preferable that the totality of injected fuel obey a progressive slope until it encounters the setpoint value, in such a way as to ensure that part of the injected fuel does not pass through the catalyst without having reacted. With such an injection profile, the reducing agents passing through the catalyst in the case of strong exhaust-gas flow and elevated wall temperature have a better chance of being oxidized.

In order to improve the system dynamics, the present invention proposes that the exhaust injector flow be varied prioritarily in response to a variation of the setpoint for total flow. In this way, the post-injection is insensitive to variation of the setpoint. However, since it is preferable to minimize the dilution due to post-injection, the invention provides for re-establishing the equilibrium (in other words, having the maximum possible flow of reducing agents in the exhaust and the minimum in the combustion chambers of the engine) by progressively increasing the flow of reducing agents in the exhaust.

The algorithm comprising the strategy of injection of reducing agents into the exhaust line is integrated into the ECU of the vehicle. The main steps of the strategy are the following:

-   -   the algorithm first determines, from a mapping, a supplementary         amount (Q_(red)) of fuel to be injected for the operating point         under consideration;     -   the temperature measurement at the DOC outlet (or at the PF         inlet) makes it possible to correct this amount of reducing         agent, in order to approach the desired temperature (setpoint         temperature) at the PF inlet (T_(DOCO)=T_(PFI));     -   the control system then manages the distribution of         supplementary fuel between the fifth injector (Q_(5inj)) and the         post-injection (Q_(poi1)) according to the characteristics of         the exhaust gases (T_(wall) and Q_(EG)). It is possible that         only the fifth injector or only the delayed injection will be in         operation.

Finally, it must be pointed out that the precision of the algorithm for calculating the wall temperature may limit the use of the proposed strategy. In fact, although it is important to be able to use the additional injector over the largest possible range of speed and load, it is also important that it not be used when the wall temperature is too low. The margin chosen for the value of T_(wall) will directly impact the accessible speed/load field. 

1-20. (canceled)
 21. A method for controlling regeneration of a depollution system including an oxidation catalyst and a filter, based on introduction of fuel into exhaust gases via delayed injections of fuel into certain combustion chambers of an engine and/or by direct injections into an exhaust line upstream from the filter by virtue of an injector dedicated to the exhaust line, as a function of temperature at an inlet of the system, wherein the introduced fuel flow is assigned to the direct injections into the exhaust line and/or to the delayed injections into certain combustion chambers according to a value of a wall temperature of the exhaust line.
 22. A control method according to claim 21, wherein the injection of fuel into the exhaust line is used only in certain ranges of operation of the engine.
 23. A control method according to claim 21, wherein the injection of fuel into the exhaust line is limited to one zone of relatively low loads and to one zone of relatively high loads of the engine.
 24. A control method according to claim 21, wherein the wall temperature is determined by a transducer.
 25. A control method according to claim 21, wherein the wall temperature is determined by an algorithm integrated into the engine calculator, as a function of parameters that include the temperature of the exhaust gases upstream from a turbine of a turbocompressor, water temperature, exhaust-gas flow, and air flow.
 26. A control method according to claim 21, wherein the fuel flow injected into the exhaust line is limited to a maximum flow, beyond which the injected fuel would not be completely oxidized therein by the oxidation catalyst.
 27. A control method according to claim 21, wherein the fuel is injected prioritarily into the exhaust line as long as the injected flow is smaller than a maximum flow that can be completely oxidized therein.
 28. A control method according to claim 27, wherein a surplus of fuel relative to the flow that can be oxidized in the exhaust line is introduced by delayed injections into the combustion chambers of the engine.
 29. A control method according to claim 21, wherein a total fuel flow is corrected at each engine operating point by a factor that depends on the difference between the filter inlet temperature and a regeneration setpoint temperature.
 30. A control method according to claim 21, wherein an engine calculator directs fuel flow into a dedicated injector of the exhaust line until a saturation level of the oxidation catalyst, before passing on surplus ordered by regeneration of the filter to delayed injections of fuel into the combustion chambers of the engine.
 31. A control method according to claim 30, wherein flow of the exhaust injector varies prioritarily in response to a variation of a setpoint for total flow.
 32. A control method according to claim 21, wherein the depollution system includes a particulate filter.
 33. A device for implementation of a method according to claim 21, comprising: an injector dedicated to the exhaust; a first temperature transducer upstream from a turbine of a turbocompressor; an oxidation catalyst; a second temperature transducer that measures the temperature at the inlet of the depollution system; the depollution system; and means for determining the wall temperature of the exhaust line.
 34. A control device according to claim 33, wherein the wall-temperature determining means includes a calculating algorithm integrated into a calculator.
 35. A control device according to claim 33, wherein the fuel injector is disposed upstream from a turbocompressor turbine.
 36. A control device according to claim 33, wherein the fuel injector is disposed downstream from the turbocompressor turbine.
 37. A control device according to claim 33, wherein the first temperature transducer is disposed upstream from the turbocompressor turbine.
 38. A control device according to claim 33, further comprising a fourth transducer for determining temperature at an outlet of the depollution system.
 39. A control device according to claim 33, wherein the depollution system includes a particulate filter.
 40. A control device according to claim 33, wherein the depollution system includes a nitrogen oxides trap. 